The invention relates to an X-ray diffractometer.
X-ray diffractometers have been used for many years for analysing perfect, nearly perfect and highly imperfect materials on a routine basis.
An early high resolution diffractometer is the double-crystal diffractometer which is illustrated in FIG. 1. An X-ray source 1 transmits X-rays to a collimating crystal 3 which directs them onto a sample 5 which can be rotated about an axis 7. The X-rays are diffracted by the sample 5 onto a detector 9. The sample is then rotated and the intensity of X-rays reaching the detector is measured as a function of the angle of this rotation. This structure is still widely used, and gives good results especially on perfect and near-perfect samples.
Unfortunately, double crystal diffractometers have a number of disadvantages. Firstly, the Bragg angle of the collimating crystal 3 has to match that of the sample 5. The matching of the collimating crystal and the sample crystal is very important and the resolution is fixed by the crystals. Accordingly, a different collimating crystal 3 is required for each new sample material.
Secondly, the X-rays in a double crystal diffractometer reach a large and ill-defined region of the sample. Since this ill-defined region is the region analysed, X-ray diffractometers do not give good results for non crystalline materials or materials with defects.
Thirdly, in a double crystal diffractometer a number of different wavelengths are diffracted into the detector simultaneously. This reduces the precision and resolution of the results.
Fourthly, there is a large background level of illumination that reaches the detector from imperfect samples which makes analysis of any results a problem.
Furthermore, the double crystal diffractometer is very sensitive to the alignment and significant changes can be observed with a tilt angle varying by only 0.2xc2x0. This makes the double crystal diffractometer difficult to set up.
A number of proposals to alleviate some of these problems have been made.
Bartels describes a diffractometer which removes the inconvenience of having to change the crystal in xe2x80x9cCharacterization of Thin Layers on Perfect Crystals with a Multipurpose High Resolution X-ray Diffractometerxe2x80x9d, Bartels, Journal Vacuum Science and Technology B Volume 1 page 338 (1983). Bartels replaced the collimating crystal of a double crystal diffractometer with two channel-cut monochromators which provide a good monochromator with a very narrow angular divergence. However, this change to the double-crystal diffractometer geometry does not solve many of the problems of the double-crystal diffractometer. In particular, the Bartels diffractometer cannot be used for interpreting complex diffraction patterns from all but the most perfect and flat samples.
Another prior art design is a triple-crystal diffractometer which overcomes the problem of a bent sample and produces a significantly improved diffraction profile. This is illustrated in FIG. 2. The triple crystal diffractometer differs from the double crystal diffractometer by having a further analysing crystal 4 interposed between the sample 5 and the detector 9. Each of the crystals is independently rotatable. The analyser acts as a detector with a very narrow angular acceptance.
Unfortunately, away from the position in which all three crystals have the same scattering angle the recorded intensity profile recorded at the detector broadens rapidly. Furthermore, alignment is very difficult and this configuration has accordingly only been used by a few research groups. Moreover, each new sample still requires a new collimating crystal.
The current state of the art commercial instrument is a multiple crystal diffractometer, in which the collimating crystal 3 of the triple crystal diffractometer is replaced by a multiple crystal collimating arrangement.
Commercial diffractometers may be used in particular for quality control. In practice it is the double crystal approach that has traditionally been used, and the very difficult alignment of the triple-axis diffractometer is normally avoided. Although the multiple crystal diffractometer is an excellent research instrument, it has not replaced the simpler double crystal diffractometer for production line control. In view of the disadvantages of the double crystal configuration mentioned above there is a need for an alternative X-ray diffractometer suitable for routine use that addresses these disadvantages.
According to the invention there is provided an X-ray diffractometer, comprising
a sample stage for mounting a sample, the sample stage being rotatable about an axis,
a double pinhole collimator for directing X-ray radiation to a sample on the sample stage,
a detector for detecting X-rays diffracted by the sample, and
an analyser crystal arranged between the sample stage and the detector to direct X-rays diffracted by the sample onto the detector,
wherein the analyser crystal and detector are rotatable about an axis that is coaxial with the axis of rotation of the sample stage.
The new instrument reverses the crystals of a conventional double crystal arrangement and provides the second crystal after the sample rather than before. This gives greater precision in the measurement of the scattered beam direction which is most crucial for coping with imperfect materials. Moreover, the arrangement removes the need for precise wavelength control of the probing beam, which is why it is possible to replace the collimating crystal of a triple-axis arrangement with a double pinhole collimator.
As will be apparent from experimental results presented later, the diffractometer according to the inventor produces results approaching, possibly in some cases even surpassing, those obtained using a multiple crystal diffractometer. This is in spite of the fact that the diffractometer according to the invention is very much simpler to set up and to use.
The analysed region of the sample is much better defined and smaller in the diffractometer according to the invention than in a conventional double crystal diffractometer. This is because the sample is placed after the pinhole collimator. This produces a spot on a sample mounted on the sample stage with a well-defined central region (the umbra) and very little X-ray intensity in the penumbra (the region surrounding the umbra). To obtain a well defined and small region using a conventional double crystal diffractometer would require either a smaller spot area or alternatively complex focusing optics. These are not required in the diffractometer according to the invention.
A further difference over a conventional double crystal diffractometer is the narrow angular acceptance of the analyser, i.e. the analyser crystal can be nearly perfect and can direct into the detector X-rays only in a narrow angular range around the correct angle of incidence.
A significant advantage of the invention is that the combination of a well defined analysed region of the sample and the narrow angular acceptance of the analyser can greatly reduce the background and diffuse scattering that can easily make analysis a problem using a conventional double crystal diffractometer. Such scatter might frequently in prior art arrangements be X-rays diffracted from imperfect regions of the sample or from X-rays diffracted along paths not of interest to the analysis. Similarly the curvature of the sample will have a smaller influence on the breadth of the recorded intensity profile because of the narrow double pinhole collimator and analyser crystal.
In particular the combination of the well defined small analysed region and the narrow angular acceptance of the analyser crystal and detector greatly improves the performance with imperfect samples. In such imperfect sample small mosaic block sizes, defects, microscopic tilts and curvature can make it very difficult to analysis any data obtained with a conventional double crystal diffractometer. Whilst a triple axis diffractometer can significantly improve the resolution such triple axis diffractometers are extremely difficult to set up and therefore less suited to fast and routine analysis for quality control applications.
The measurement of the scattering angle is very robust. Even moving the sample out with respect to the sample stage so that the sample is one millimeter away from the optimal position gave a reproducibility of 0.0001xc2x0 in the measured scattering angle on a 004 reflection of a high quality silicon sample. Even with such a grossly misaligned sample the value of the lattice perimeter was measured to be 5.43104 xc3x85 which is within 1.8 ppm of the accepted value. Measurement of a slightly damaged piece of silicon gave a measurably different value of 5.43113 xc3x85.
A further benefit of the invention is that the diffractometer is not particularly sensitive to alignment. Tilting the sample does not give rise to significant loss of quality. This occurs because the analysing crystal can be precisely aligned and the X-rays hitting the sample are less precisely aligned than when using a collimating crystal and so the analysis is less sensitive to inaccuracies in alignment. Thus, in many cases experiments may be carried out without any tilt alignment.
The insensitivity of the diffractometer according to the invention both to tilt angle and to exact sample alignment makes the diffractometer easier to use, and hence can speed up analysis.
The analyser crystal and detector are preferably rotatable about an axis that is coaxial with the axis of rotation of the sample stage. The measured analyser angle is then the true scattering angle.
The use of the analyser crystal also significantly improves dispersive effects caused by the plurality of wavelengths in the X-ray source. This means that the same analyser crystal can be used for a whole range of experiments. This is not possible with a conventional double crystal diffractometer since the sample axis is rotated and all wavelengths that are scattered from the collimating crystal may arrive at the detector.
If the discrimination is required to be improved a small slit may be placed in front of the analyser crystal or in front of the detector. By placing the discrimination far from the source in this way a larger separation of angles for different wavelengths is obtained which makes it much easier to discriminate between those different wavelengths than would be possible in prior approaches.
The pinhole size may be adjustable which allows the resolution to be changed depending on the sample. This change in pinhole size can also allow the adjustment of the area of the sample analysed. A small spot size can allow the resolution to be roughly good as with a multiple crystal diffractometer configuration. Larger spot sizes can increase x-ray intensity and hence measurement speeds. Such adjustment is particularly useful for quality control of discrete devices.
The X-ray diffractometer may further include a drive to rotate the sample stage and the analyser crystal and detector in a 1:2 ratio. Each stage may be separately driven under computer control.
In another aspect there is provided a method of X-ray diffractometery, comprising
directing X-rays through a double pinhole collimator onto a sample to be measured,
diffracting the X-rays diffracted by the sample from an analyser crystal onto a detector,
rotating the sample and rotating the analyser crystal and detector about coaxial axes, and
measuring the diffracted X-ray intensity as a function of the angle of rotation of the sample and the angle of rotation of the analyser crystal and detector.
The method may include varying the size of at least one pinhole in the double pinhole collimator.
It is possible to measure the scattering angle to within 0.0001xc2x0 but it is inconvenient to have to align a sample on the sample stage to this very high level of accuracy. Accordingly, the method may include the steps of
mounting the sample on a sample stage;
rotating the analyser crystal and detector to a predetermined position;
rotating the sample whilst keeping the analyser crystal and detector in the predetermined position and measuring the X-rays reaching the detector as a function of angle of sample rotation; and
determining the sample rotation angle at which the measured X-rays are at a peak and rotating the sample to that angle.
In this way the sample can be mounted on the stage without needing accurate alignment and the sample can then be aligned by rotating the sample to the angle wherein an X-ray peak is measured, without having to align the sample to the sample stage with extreme accuracy.
The method may further comprise the step of rotating the sample and the analyser crystal and detector with rotation speeds substantially in a 1:2 ratio.